Diffusion

Non-polar molecules such as fatty acids, oxygen and carbon dioxide are able to dissolve directly through the membrane and enter the cell by diffusion. This process is used in the lungs whose function is the gas exchange of carbon dioxide and oxygen across the epithelium of the alveoli. Contraction of the intercostal muscles and the flattening of the diaphragm move the rib cage up and out, increasing the volume of the thorax. This decreases the pressure allowing air to be drawn into the lungs down a pressure gradient. This ventilates the epithelial cells of the alveoli allowing oxygen to diffuse through the membrane through the cells. The oxygen then continues to diffuse through the membrane of the red blood cells where it loads to haemoglobin forming oxyhaemoglobin. The carbon dioxide follows the reverse route and is expelled from the lungs during expiration as the intercostal muscles relax.